In a drive apparatus (optical disk apparatus) which is capable of performing optical recording/reproduction of information for a disk-shaped optical disk, focusing control and tracking control are carried out so as to cause the focal point of a light beam to be placed at a desired position on the recording surface of a rotating optical disk, by using a spindle motor or the like. In an optical disk apparatus which is capable of performing recording/reproduction of information for an optical disk such as a CD-R or a CD-RW, tracking control based on a differential push-pull (Differential Push-Pull: DPP) technique is performed. A DPP technique generates a tracking error signal by applying calculations to output signals from respective photodetectors, which are obtained from a main beam and two sub beams.
Hereinafter, with reference to FIG. 1, a DPP technique which is performed in the aforementioned optical disk apparatus will be specifically described. FIG. 1 shows the structure of an optical system 10 in an optical pickup of an optical disk apparatus. In this optical system 10, a diffraction grating 202 is disposed in a forward path of a light beam which is emitted from a laser light source 201. The diffraction grating 202 diffracts the light beam which is emitted from the laser light source 201 to generate three beams of light, i.e., 0th order diffracted light (main beam) and two beams of 1st order diffracted light (sub beam). The above three beams of light which have been generated through diffraction by the diffraction grating 202 form three light spots on an optical disk 206 via a beam splitter 203, a collimating lens 204, and an objective lens 205. Light which has been reflected by the optical disk 206 is received by a photodetector 208 via the beam splitter 203 and a detection lens 207.
Now, with reference to FIG. 8, relationship between the positions of spots of the three beams of light formed on the optical disk 206 will be described. FIG. 8(a) is a plan view schematically showing a relationship of spot positions in a state where the optical disk 206 is not tilted with respect to the optical system 10 of FIG. 1. For reference's sake, a partial cross section of the optical disk is shown below the plan view.
As can be seen from FIG. 8(a), a spot of the main beam 30 is formed on a predetermined recording track among a plurality of recording tracks. On both sides of the recording track which is followed by the main beam 30, spots of the sub beams 32 and 33 are formed. More specifically, spots of the sub beams 32 and 33 are positioned near the centers of the guide tracks which are on both sides of the recording track on which the spot of the main beam 30 is positioned. As a result, the positions of the spots of the sub beams 32 and 33 on the optical disk, along the radial direction, are shifted with respect to the position of the spot of the main beam 30 by ±0.5 track pitches.
FIG. 2 shows a detailed structure of the photodetector 208. As shown in FIG. 2, the photodetector 208 includes a main-beam photodetector 301 which is irradiated with the main beam 30 having been reflected from the optical disk 206, and sub-beam photodetectors 302 and 303 which are respectively irradiated with the sub beams 33 and 32 having been reflected from the optical disk 206. Through photoelectric conversion, the photodetector 208 outputs electrical signals which are in accordance with the intensity of the light received by each detection section.
The main-beam photodetector 301 is split in four: detection sections 301a, 301b, 301c, and 301d. The sub-beam photodetector 302 is split in two: detection sections 302e and 302f. The sub-beam photodetector 303 is split in two: detection sections 303g and 303h. 
The split detection sections 301a, 301b, 301c, 301d, 302e, 302f, 303g, and 303h output signals A, B, C, D, E, F, G, and H, respectively. By subjecting these signals A to H to calculations, tracking servo error signals are generated. Specifically, based on the signals A to D which are output from the main-beam photodetector 301, an MPP calculation circuit 304 generates a main push-pull signal (MPP). Based on the signals E to H which are output from the respective sub-beam photodetectors 302 and 303, an SPP calculation circuit 305 generates a sub push-pull signal (SPP) and a DPP calculation circuit 306 generates a differential push-pull signal (DPP).
The aforementioned calculations performed by the MPP calculation circuit 304, the SPP calculation circuit 305, and the DPP calculation circuit 306 are executed in accordance with (eq. 1), (eq. 2), and (eq. 3) shown below.MPP=(A+D)−(B+C)  (eq. 1)SPP=SPP1+SPP2=(F−E)+(H−G)  (eq. 2)
                                                        DPP              =                              MPP                -                                  α                  ×                  SPP                                                                                                        =                                                (                                      A                    +                    D                                    )                                -                                  (                                      B                    +                    C                                    )                                -                                  α                  ×                                      {                                                                  (                                                  F                          -                          E                                                )                                            +                                              (                                                  H                          -                          G                                                )                                                              }                                                                                                          (                  eq          .                                          ⁢          3                )            
Herein, α is a constant which is determined based on the intensities of the 0th order diffracted light, +1st order diffracted light, and −1st order diffracted light. Although eq. 3 includes the coefficient a, the differential push-pull signal (DPP) is, in the broad sense of the word, a differential signal between the main push-pull signal (MPP) and the sub push-pull signal (SPP).
According to the aforementioned tracking servo method, as shown in FIG. 2, the positioning of optical elements such as the diffraction grating 202, the laser light source 201, the photodetector 208 is set so that the respective beams will be positioned on the centers of the split lines of the photodetectors 301, 302, and 303.
FIG. 3 shows signal waveforms 401, 402, and 403 of the main push-pull signal (MPP), the sub push-pull signal (SPP), and the differential push-pull signal (DPP) in the case where the aforementioned ideal positioning is realized.
As is clear from FIG. 3, the phase of the SPP waveform 402 is shifted by π rad (180°) with respect to the phase of the MPP waveform 401, the two waveforms being of inverted relationship. Such a relationship is obtained because, as shown in FIG. 8(a), the spots of the sub beams 32 and 33 are positioned not on recording tracks but on guide tracks, so that their signal polarities are inverted.
Since the polarity of the SPP waveform 402 and the polarity of the MPP waveform 401 are opposite, the phase of the DPP waveform 403 which is obtained in accordance with eq. (3) has the same phase as that of the MPP waveform 401.
In the case where the optical disk 206 is not tilted, as shown in FIG. 8(a), the light spot of the main beam 30 upon the optical disk 206 is on a track center at the position indicated by reference numeral “40” in FIG. 3. The DPP waveform 403 is calibrated and set so as to indicate a zero value at this time.
In the DPP technique, there is performed a tracking control for causing the entire objective lens or the entire optical pickup device to move along a radial direction of the optical disk 206 in such a manner that the DPP waveform 403 exhibits a zero value. Since the light spot that is the target of the tracking control is the light spot of the main beam, the light spot of the main beam will be abbreviated as the “light spot” in the following descriptions, for simplicity.
The above-described conventional optical pickup device is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2001-307351.
In the case where the optical disk 206 or the objective lens 205 is tilted along a radial direction of the optical disk, the MPP, SPP, and DPP signal waveforms change to an MPP waveform 501, an SPP waveform 502, and a DPP waveform 503, respectively, as shown in FIG. 4. This is because, if the optical disk 206 is tilted as shown in FIG. 8(b), the main beam 30 and the sub beams 32 and 33 will be obliquely incident on a recording track/guide tracks on the optical disk 206. As a result, a phase difference occurs between the MPP waveform 501 and the SPP waveform 502. Assume that a phase difference of φ emerges between the MPP waveform 501 and the SPP waveform 502. In this case, the phase of the DPP waveform 503 is shifted from the phase of the ideal signal waveform, which would exhibit a zero value when the light spot is on a track center, and has a phase difference with a magnitude of “φ”. Therefore, when a tracking control is performed based on such a DPP waveform 503, the DPP waveform 503 will indicate a zero value at a position indicated by reference numeral “51” in FIG. 4, and therefore the actual light spot will be controlled to a position which is shifted by a distance Δ, which corresponds to the phase difference φ, from the track center (i.e., a position indicated by reference numeral “50”). This distance Δ will be referred to as an “off-tracking amount” of the light spot position. Although the off-tracking amount Δ is described in relation to the DPP waveform 503 in FIG. 4, the actual off-tracking amount is a distance between the light spot position of the main beam and a recording track center on the optical disk.
With the DPP signal (tracking error signal) which has incurred a phase shift as above, it is impossible to control the position of the light spot to be accurately on a track center, so that the tracking control becomes unstable. This results in an off-tracking, i.e., deviation of a light spot on an optical disk from a track center, whereby the recording/reproduction characteristics of the optical disk apparatus are deteriorated.
The present invention has been made in order to solve the above problems, and is aimed at providing: an optical pickup device capable of correcting an off-tracking which is ascribable to a phase shift of a DPP signal waveform, even when an optical disk or an objective lens is tilted along a radial direction of the optical disk, so that stable tracking control can be performed; and, an optical disk apparatus comprising such an optical pickup device.